订阅最近很長一段時間都在優化一個項目，這個項目存在許多問題，從數據架構到工作流程，我一直在思考一些有關oracle性能優化的問題，我跟開發實施人員進行過多次交流，發現存在許多交流障礙，許多問題實施人員覺得sql語句執行很快，並不存在性能問題，我花了大量時間要他看執行計劃，說明當數據積累到一定時間後，會執行越來越慢。
我舉一個例子來說明問題：
SELECT v2.*
FROM consultationrecorddoctor t1
RIGHT OUTER JOIN
(SELECT cr.*, crc.modifydate, crc.modifyuserid, crc.consultationtime,
crc.applyconsultationdeptid, crc.askconsultationdeptid,
crc.consultationdeptid, crc.casehistory_right,
crc.consultationorder_right, crc.consultationidea_right,
crc.otherhospital_right, crc.consultationdate,
crc.maindoctorid, c1.deptname_vchr AS applydeptname,
c2.deptname_vchr AS askdeptname,
c3.deptname_vchr AS deptname,
f_getempnamebyno (crc.maindoctorid) AS maindocname
FROM consultationrecord cr,
consultationrecordcontent crc,
t_bse_deptdesc c1,
t_bse_deptdesc c2,
t_bse_deptdesc c3,
(SELECT cr.inpatientid, cr.inpatientdate, cr.opendate,
cr.createdate,
MAX (crc.modifydate) AS maxmodifydate
FROM consultationrecord cr,
consultationrecordcontent crc
WHERE (crc.applyconsultationdeptid = '0000208')
AND cr.inpatientid = crc.inpatientid
AND cr.inpatientdate = crc.inpatientdate
AND cr.opendate = crc.opendate
AND cr.status = 0
GROUP BY cr.inpatientid,
cr.inpatientdate,
cr.opendate,
cr.createdate) v1
WHERE (crc.applyconsultationdeptid = '0000208')
AND cr.inpatientid = crc.inpatientid
AND cr.inpatientdate = crc.inpatientdate
AND cr.opendate = crc.opendate
AND cr.status = 0
AND crc.applyconsultationdeptid = c1.deptid_chr
AND crc.askconsultationdeptid = c2.deptid_chr
AND crc.consultationdeptid = c3.deptid_chr
AND cr.inpatientid = v1.inpatientid
AND cr.inpatientdate = v1.inpatientdate
AND cr.opendate = v1.opendate
AND v1.maxmodifydate = crc.modifydate
AND crc.maindoctorid IS NOT NULL) v2
ON t1.inpatientid = v2.inpatientid
AND t1.inpatientdate = v2.inpatientdate
AND t1.opendate = v2.opendate
AND t1.modifydate = v2.modifydate
AND t1.employeeflag = 1
WHERE t1.employeeid IS NULL
ORDER BY t1.employeeid DESC, v2.askdeptname, v2.consultationdate;
查詢使用了一個右連接，查詢的表並沒有包含t1表的字段，而且在實際的t1.employeeid中
根本不存在NULL的值，實際上是從一個大的結果集剔除一個大結果集的而獲得一個小的結
果集，隨著crc表數據量加大，會越來越慢！
